1. Field of the Invention
The present invention relates to cubic boron nitride (hereinafter referred to as CBN) employed in a grinding wheel or a similar device; to a CBN synthesis catalyst (hereinafter may be referred to as solvent) employed for producing CBN from hexagonal boron nitride (hereinafter referred to as HBN); and to a method for producing CBN by use of the CBN synthesis catalyst.
2. Background Art
CBN is second to diamond in hardness and has chemical stability higher than that of diamond. Thus, cubic boron nitride is increasingly employed as abrasive grains for producing grinding material, polishing material, or cutting material. A variety of methods for producing cubic boron nitride have been proposed. Among them, best known and widely employed in the industrial field is a method in which HBN is maintained in the presence of a CBN synthesis catalyst substance under conditions where cubic boron nitride remains thermodynamically stable (approximately 4 to 6 GPa, approximately 1,400 to 1,600° C.), to thereby transform hexagonal boron nitride into cubic boron nitride (see, for example, Patent Documents 1 to 7). CBN synthesis catalysts known to be employed in the above production include an alkali metal, an alkaline earth metal, an alkali metal nitride, an alkali metal boronitride, an alkaline earth metal nitride, an alkaline earth metal boronitride, and other similar compounds, and these compounds are used singly or in combination of two or more species. Since these CBN synthesis catalysts are usually added to HBN serving as a starting material, the catalysts are generally used in the form of powder, agglomerates, etc. so as to readily form a mixture with HBN.
These CBN synthesis catalysts, having high reactivity with oxygen, water, or a similar substance, are readily deteriorated to form substances such as oxides, hydroxides, or carbonates. As has been known, when the catalytic performance of the catalysts is thus deteriorated, characteristics of formed CBN, its production yield, and other properties are adversely affected. Therefore, these CBN synthesis catalysts are handled with the utmost care in a chamber such as a gloved box where dry nitrogen gas flow or similar gas flow is maintained so as to prevent deterioration or denaturation of the CBN synthesis catalysts caused by contact with water moisture, oxygen, carbon dioxide, etc.
As has also been known, deterioration of these catalysts is promoted by oxygen or water contained in raw material during synthesis of CBN. In this connection, there has been disclosed a method for preventing deterioration of a CBN synthesis catalyst including, for example, removing an oxygen source (predominantly containing oxides such as boron oxide) contained in starting HBN (see, for example, Patent Documents 8 to 13). These documents also disclose a method for removing an oxygen source such as boron oxide contained in starting HBN during synthesis, which method includes adding a carbon source to the starting material, and a method for removing an oxygen source, which method includes reducing starting HBN before performing synthesis of CBN; heating starting HBN to which a carbon source has been added; or a similar step.
Through any of these methods, the oxygen source contained in starting HBN is decreased, thereby preventing deterioration of the catalyst, leading to enhanced CBN characteristics and improved production yield.
Patent Document 1    Japanese Patent Application Laid-Open (kokai) No. 58-84106 (U.S. Pat. No. 4,551,316)
Patent Document 2    U.S. Pat. No. 2,947,617 specification
Patent Document 3    Japanese Patent Application Laid-Open (kokai) No. 59-57905
Patent Document 4    Japanese Patent Application Laid-Open (kokai) No. 59-73410
Patent Document 5    Japanese Patent Application Laid-Open (kokai) No. 59-73411
Patent Document 6    Japanese Patent Application Laid-Open (kokai) No. 59-18105
Patent Document 7    Japanese Patent Application Laid-Open (kokai) No. 2002-284511 (U.S. Pat. No. 2003-170,161 A1)
Patent Document 8    Japanese Patent Application Laid-Open (kokai) No. 2-35931 (U.S. Pat. No. 5,000,760)
Patent Document 9    Japanese Patent Application Laid-Open (kokai) No. 2-36293 (U.S. Pat. No. 5,000,760)
Patent Document 10    Japanese Patent Application Laid-Open (kokai) No. 2-233510
Patent Document 11    Japanese Patent Application Laid-Open (kokai) No. 2-115034
Patent Document 12    Japanese Patent Application Laid-Open (kokai) No. 59-217608
Patent Document 13    Japanese Patent Application Laid-Open (kokai) No. 1-168329
Patent Document 14    Japanese Patent Application Laid-Open (kokai) No. 58-120505
Patent Document 15    Japanese Patent Application Laid-Open (kokai) No. 48-55900
Patent Document 16    Japanese Patent Application Laid-Open (kokai) No. 3-80929 (EP No. 0,402,672 A)
Patent Document 17    Japanese Patent Application Laid-Open (kokai) No. 5-146664 (EP No. 0,512,762 A)
As is described in the above disclosed methods, CBN grown inhibitors such as boron oxide contained in the starting HBN are removed through addition of a carbon source to HBN, thereby attaining effects such as ready production of crystals having sharp edges or large crystals by virtue of reduced inhibition of development of crystal plane; ready formation of CBN nuclei; and enabling synthesis under milder conditions. In other words, the carbon source employed in these methods is considered to provide the effect of removing impurities contained in starting HBN and to provide the effect of enhancing characteristics and production yield of CBN.
However, even when the impurities contained in the starting HBN have been removed, the strength of the formed CBN is not sufficiently enhanced, and drop in strength caused by heating remains large. Such an unsatisfactory strength is conceived to be attributed to a catalyst component incorporated into CBN during synthesis of CBN. As has been also reported (see, for example, Patent Documents 14 to 17), incorporation of the added carbon source makes CBN more breakable.
Therefore, when a grinding wheel is fabricated from such a CBN product, CBN particles are readily worn out or broken during use of the grinding wheel under severe process conditions such as high load grinding. Thus, CBN having high strength and high heat resistance has been keenly demanded. Needless to say, although conventional CBN synthesis catalysts to which no carbon source has been added exhibit no drop in strength of CBN crystals—such a drop in strength being caused by incorporation of the carbon source—the conventional catalysts exert no effect of removing a CBN growth inhibitor contained in starting HBN, and desired improvement in characteristics and production yield of CBN may fail to be attained due to the presence of a growth inhibitor, incorporation of impurities into CBN, or other factors.